What is everyones preferred fleet size?
I like a fleet of 4 or 5 ships but sometimes I am unable to get this as, even though my available credits have increased with missions completed, the cost of the 'minimum available ship' has increased such that I can't afford a fifth ship.
Is it ever possible to get a fleet of seven ships?

Many small ships is more desirable than just a few large ships. I've played both strategies and found that I could easily get over 100 missions so long as I kept using a fleet of small ships. As mentioned above, after you get a large amount of money, the smaller ships no longer show up on the selection screen.

I like about 5 ships. I use stealthy ships that are small and very, very hard for the enemy to pick up. But they carry 4 radars in front so we tend to make a mess of unsuspecting ships. I prefer my standard stealth fighters to fly on my wing. They are 116 creds. then there is the heavy version that is 150 creds. Then the advanced one, which is 167 creds. They are all fairly small and tend to do great together for the most part.

The weighting of the cost appears to be biased towards having fewer ships. I frequently have only sufficient funds to buy less than 4 of the median ship provided. Could the be because I have designed quite a few 'more expensive' ships?

I tend to have a team of very small weaklings if I'm being evil, but generally I favor strong ships over a large fleet of weaker ones.

It's possible to get 7 good ships really quick, as long as you stay alive; I managed to get 7 Warrior equivalents (Better, for all super shields on all sides) at the cost of 424 each near mission 400something.

I make sure to get the ship of choice for myself first, and I find I also tend for quality over quantity in that I choose high-end ships (usually custom design, scouts with a bunch of Drachans and moderate armor) for allies first, since I prefer that they not die in the middle of a battle.